The present invention relates to a coating for cables, which is capable of limiting the absorption of moisture by the cable.
Cables, in particular those for fiber optics telecommunications, have the disadvantage of the possible ingress of water, either in the liquid form (for example, phreatic water in the case of underground cables, or sea water in the case of submarine cables), or in vapor form (for example, atmospheric humidity), as a result of discontinuities due to faults or defective joints in the outer coating and, in particular in the case of cables without outer metal sheaths, as a result of the incomplete impermeability of the outer coating of the cable.
In particular, the optical fibers located inside such cables may be damaged by operation in conditions of high relative humidity (hereinafter termed "r.h."), for example for values of r.h. in excess of 80%. The main problem arising from the contact of the optical fibers with water is what is known as "delamination", in other words a possible detachment from each other of the various layers forming the coating of the glass fiber. These layers, which are generally polymer compounds based on acrylates or urethane-acrylates, are normally identified, starting with that in contact with the glass, as the primary coating, the secondary coating, the ink, and, in the case of a number of optical fibers joined to form a "ribbon", the common coating. The typical phenomena of delamination are those occurring between the glass and the primary coating of the fiber, between the ink and the secondary coating, and between the common coating and the ink.
Such delaminations may, on one hand, cause attenuation in the transmission of the signal, and, on the other hand, decrease the mechanical strength properties of the fiber.
Said problems are of particular significance for non-sealed cables, in other words those without welded or extruded metal outer sheaths, and in particular for what are known as "all-dielectric" cables, in other words those having external protection consisting solely of a polymer coating.
There are various documents which envisage the possibility of inserting into the cable particular compounds or materials capable of absorbing water which has penetrated accidentally into the cable.
For example, International Patent Application WO-A-88/10000 describes the use of a synthetic expandable powder substance in which the rapidity of inflation due to the absorption of water is obtained by reducing the particle size of the inflatable substance.
European Patent Application EP 714042 describes a packing containing inflatable moisture-absorbent polymer powders.
Patent GB 2165368 describes a solution for preventing the ingress of moisture into a fiber optics cable, consisting in filling the interstices of such a cable with a packing compound containing a desiccant material, in particular a zeolite.
Such documents therefore relate to solutions which envisage an absorption of water inside the cable, without consideration of the permeability of the outer coating. In general, in order to ensure satisfactory water absorption in the absence of a sealed metal coating, it is necessary to use considerable quantities of absorbent material, with consequent manufacturing problems. The applicant has also observed that, in the presence of large quantities of absorbed water inside the cable, a release of vapor is possible as a result of heating cycles.